On: Rewrite It in Rails

[MrJohz]

It's not an explicit feature, but a combination of arbitrary records, sum types, and type narrowing:

    type Animal =
      | { species: "dog", name: string, catsChased: number }
      | { species: "cat", name: string }

    const animal: Animal = ...

    // Available in all cases
    console.log(animal.name)

    if (animal.species === "dog") {
      // Can access dog attrs here
      console.log(animal.catsChased)
    }

    // ERROR: catsChased only exists for dog and not for cat
    animal.catsChased